Later in their statement, as reported in the Independent, they explain their predicament. "The historic underfunding of the visual arts," they say, "is causing the galleries to be financially unstable." We get the message. But why did they put "historic"? One often hears people speaking of "this historic occasion", perhaps when laying a foundation stone, when they mean that they expect the building in question to become important enough in due course to be written about in the history books, though they can't be sure yet. Grammarians call this the proleptic use. (Prolepsis: "a term in rhetoric for treating a future event as though it has already happened.")
The one thing the directors don't want is for their underfunding to become historic. All they are really saying is that it has been around for a long time, like an ancient monument. I think the word they were groping for must have been chronic, which comes from the Greek for time and means "longstanding", except when doctors use it as a euphemism for "incurable", which again is not what the art galleries are hoping for. With any luck, and a more sympathetic government, their cash shortage will be "ancient history", the thing about ancient history being that everyone has forgotten it, whereas children learn about historic events in schools and these are supposed to be remembered by all.
History also came from the Greeks, whose word historia meant "investigation" and later "narrative", but not necessarily true narrative; historikos was their word for a storyteller. Herodotus, the first serious historian, was a bit of a storyteller himself. The word story was simply a shortening of history. The two words were being used interchangeably well into the 18th century; people would talk about "The True Story" of this or that to distinguish it from the nursery or bedtime sort. Even now, if you are rash enough to ask someone to tell you the story of his or her life, you expect a true account, and the "true" in True Love Stories (as told in strip form) presumably refers to the stories, not to the love.
The muddle about what history meant was not resolved (if it was resolved at all) till it became accepted as being about what actually happened, rather than about what the historians and the balladmongers claimed to have happened. It then became a subject, where before it had only been a literary form. For great historians it was always both. John Bagnall Bury, who edited The Decline and Fall of the Roman Empire in seven volumes in about 1900, accused Edward Gibbon of occasionally being a "misleading guide" and said that some of his history was "entirely false"; plenty of people agreed with him. The 1910 Britannica defended Herodotus from the charge of being a liar, but was particularly severe on Suetonius, calling his Lives of the Caesars "but a superior form of journalism" - a gratuitous insult to the profession (journalism, not history).
Historical truth can hardly be anything but a comparative term. Historic events, on the other hand, have always been immutably "true". But then words have never been the artist's medium.